Topband: Ground screen ????

[Jim Brown]

On Tue,3/1/2016 7:29 PM, Brad Rehm wrote:
> You certainly can use the galvanized roof as a ground screen.

Certainly can is FAR too optimistic a statement. Poor connections 
between segments of the roof will do more than cause SWR shifts, they 
will create intermod, which could make you unpopular in nearby places 
like FL. Also, the roof, 16 ft above ground, shortens the radiator by 
that amount.

>   It might be
> a good idea to measure resistance between sheets to get an idea of how well
> they're all tied together electrically.  If they're not, you might see some
> odd shifts in VSWR when you send RF through it.  Another potential problem
> is that RF may be coupled into the building wiring when you transmit.

That's likely to happen with ANY vertical that is next to the building, 
but the problem will be minimized if we give the antenna a good radial 
system, ground screen, or counterpoise.

> This is why you'll want to bring grounds up to the roof in as many places as you
> can.

Gee, misconceptions keeps getting perpetuated!  A connection to the 
EARTH does NOT make an antenna work better. We use radials, ground 
screens, and counterpoises to SHIELD the antenna and it's field from the 
earth, NOT to make a connection to it.  We want the antenna field and 
drive current to return to the radials or ground screen or counterpoise, 
not the lossy earth. The ONLY reason for an earth connection is 
LIGHTNING protection (and that's a very good reason).

Almost 40 years ago, Rob Sherwood showed that galvanized screen (like 
fencing) can be an effective ground screen when the space for radials is 
very limited. I described this in considerable detail in this slide 
presentation. I suggest that you begin by studying the entire slide 
show. The ground screen discussion begins with slide #55.  
http://k9yc.com/160MPacificon.pdf

73, Jim K9YC